City and suburban water systems have an extensive network of main and branch pipe lines, and there are fire hydrants located at intervals along them. It is good practice to flush these lines periodically to remove accumulated sediments for public health reasons and to exercise the hydrant valves to insure proper working order. This requires opening each fire hydrant in turn and allowing it to flow wide open to flush the lines with high velocity flow until the water runs clear.
An uncontrolled stream of water thrown by a fire hydrant has so much force it can tear up boulevard turf and break or strip nearby shrubs that it may hit. If the direction of discharge cannot be controlled it is possible to cause damage to nearby driveways and low lying basements. Occasionally small rocks amd other solid objects find their way into water mains, especially in new construction areas, and when thrown by a discharging hydrant they can strike cars or people and cause property damage or personal injury.
One response to the above problems has been to connect a short length of fire hose to the hydrant and run it to a street gutter nearby. However, the water blasting out of the hose may kick up loose pebbles or pavement in the gutter, and the reaction force of the water flow makes the free end of the hose difficult and possibly hazardous to control.
Devices are available to reduce the force of the hydrant discharge. U.S. Pat. Nos. 4,047,668 and 4,343,435 disclose diffusers which can be screwed directly onto a hydrant outlet. These direct the water flow straight into a diverging nozzle which contains diffusing elements and has a relatively large outlet area. These serve to slow the velocity of the water flow and thus reduce its force. However, they have little or no provision for controlling the direction of the discharge, and they drop the water on the ground adjacent to the hydrant, so it may wash over the boulevard or private property.
Other diffusers are available which are mounted on a pickup truck or other work vehicle. The truck can be parked in the street near a hydrant and the diffuser can be connected to the hydrant by a hose. These diffuse the water and discharge it into the street gutter, but they provide no selection as to the direction of the discharge, so it may strike a car parked along the curb.